Positron Emission Tomography (PET) Scan

January 15, 2026

This page was reviewed under our medical and editorial policy by Henry Krebs, M.D., associate clinical professor, Department of Diagnostic Radiology, City of Hope^® Cancer Center Atlanta 

During cancer treatment, it is essential for a patient’s health care team to monitor the status of their cancer and his or her response to therapies. To do this, the care team often orders various imaging tests during the diagnosis, staging and treatment process.

Positron emission tomography (PET) scanning is a common type of scan used during cancer diagnosis and treatment. Here’s what patients and their families need to know about how doctors use PET scans, how to prepare for a PET scan and potential side effects.

What Is a PET Scan?

A PET scan is a special imaging test that helps doctors see how the organs and tissues inside the body are functioning. This scan uses a slightly radioactive substance called a tracer, commonly fluorine-18 (18F) fluorodeoxyglucose (FDG), to measure the activity of cells in the body. Some providers might also call this type of scan an FDG-PET scan.

This test is valuable for identifying areas of high metabolic activity typical in many cancers. Cancer cells grow rapidly, so they need glucose (sugar) for fuel more quickly than normal cells. The FDG tracer is a sugar-like substance. Fast-growing cancer cells take up the FDG and then show up as bright spots on the PET scan.

This tool is very helpful in diagnosing cancer, determining the stage of the cancer and monitoring treatment success. PET scans are especially useful in cancers like Hodgkin lymphoma, non-Hodgkin lymphoma and lung cancer. They are also used to detect whether cancer has spread to other parts of the body.

PET Scan Versus CT Scan

A computed tomography (CT) scan is another type of specialized scan. It takes X-rays of the body from multiple angles, which a computer then combines to create detailed 3D images of the inside of the body.

While a PET scan shows unusual activity, a CT scan shows detailed pictures of organs and tissues. In some instances, a PET scan and a CT scan are done at the same time (a PET-CT scan). A combination of PET and CT scans helps health care providers:

• Determine the stage of a cancer
• Locate the correct place to perform a biopsy
• Judge the success of cancer treatment
• Plan a course of radiation therapy
• Monitor for new cancer that returns after treatment

PET Scan Prep

Getting ready for a PET scan does require some preparation. When the patient schedules the scan, the staff will provide some instructions. It is important to follow them to obtain the most accurate results possible.

Diet: The patient may be asked to follow a low-carbohydrate, no-sugar diet for about 24 hours before the test. This helps the sugar-like tracer be absorbed into the cells. The patient should avoid eating or drinking anything for about six hours before the scan.

Activity: Patients should avoid any strenuous exercise, such as running or weightlifting, for about 24 hours before the test. This may increase tracer uptake in the body’s tissues and affect scan results.

Attire: Patients should wear loose-fitting, comfortable clothes on the day of the test. They will need to remove all metal before the PET scan. This includes zippers, buttons, jewelry and glasses. Patients may be asked to wear a hospital gown for the scan.

Patients should also make sure to tell the staff about any medical conditions (especially diabetes), allergies, and medications or supplements. They may also be given special instructions for these.

PET Scan Procedure

Most PET scans take place in the nuclear medicine or radiology departments of a hospital.

At the appointment for a PET scan for cancer, staff will start an IV (intravenous) in the patient’s vein. They will then administer the radioactive tracer. The patient will be asked to sit quietly for 30 to 90 minutes to allow the tracer to spread throughout the body.

When it’s time for the scan, the technologist will assist the patient onto a padded table and get into position. The position depends on which areas of the body are being scanned and may be adjusted during the test.

During the test, the padded table will move back and forth through a scanner shaped like a large doughnut. The patient needs to be very still during this time.

After the PET scan, patients may resume their normal diet and activities. Patients will likely be told to drink several glasses of water throughout the day. This will help wash the radioactive tracer out of the body.

How Long Does a PET Scan Take?

In general, an appointment for a PET scan may last up to three hours. However, much of this will be spent waiting. After the patient checks in and has the tracer injected into a vein, he or she will need to sit for 30 to 90 minutes while the tracer spreads through the body.

The actual PET scanning procedure only takes about 20 to 30 minutes to complete.

PET Scan Side Effects

Positron emission tomography scans are considered generally safe. There is a small exposure to radiation due to the tracer used in the scan. The amount of radioactive material used is very small, and the exposure is roughly about the same as that from natural sources such as the sun.

Providers order PET scans because they judge the benefits to be greater than the potential side effects. Patients should make sure that their care team is aware if they have had multiple imaging scans. Providers may take steps to further lower the risks of radiation exposure, such as using other types of scans or limiting the areas of the body that are scanned.

Additionally, some patients may experience redness, swelling or pain where the needle was inserted for the IV.

Are Patients Radioactive After a PET Scan?

Directly after the PET scan, patients will be very slightly radioactive. However, this wears off quickly and typically does not pose a danger to anyone else.

The radiation dosage used is very small and has a low risk of being toxic. Health care staff will instruct the patient to drink several glasses of water to help flush the radioactive tracer out of the body.

PET Scan Results

Patients will not receive the results of their PET scan at the time of the appointment. After the test is done, a doctor who specializes in interpreting these types of scans, such as a nuclear medicine specialist or radiologist, will review the images. The results of the PET scan will usually be available to the patient’s care team within a few days.